Vector magnetic sensors have become increasingly popular for use as electronic compasses in consumer devices such as cellular phones and automobile navigation systems, and for various applications involving positioning and measurement. These devices need to consume little power and can be produced at low cost and in high volume for consumer electronics.
There are various means by which the magnetic signal could be detected for a vector magnetic sensor application, and of these, there are many magnetic sensing technologies that can be integrated into a semiconductor chip. These include Hall Effect Sensors or magnetoresistive sensors including anisotropic magnetoresistance (AMR, Anisotropic Magnetoresistance) and giant magnetoresistance (GMR, Giant Magnetoresistance). Hall Effect devices are comparatively expensive and lacking in resolution. They are generally sensitive to fields oriented perpendicular to the plane of the substrate onto which they are fabricated. AMR and GMR devices although they are relatively high-resolution devices, suffer from low signal amplitude and require careful attention to be paid to back-end electronics design, which increases system complexity and size and therefore increases cost. AMR and GMR sensors are generally sensitive to fields parallel to the plane of the substrate onto which they are deposited.
MTJ sensors detect the magnetic field through the use of the tunneling magnetoresistance (TMR, Tunneling Magnetoresistance) effect, offer small size, high resolution, and large signal amplitude. These features can be used to simplify electronics design, thereby lowering total system cost. Like AMR and GMR sensors, MTJ sensors are sensitive to fields parallel to the plane of the substrate onto which they are fabricated.
As with most semiconductor devices, the best means to achieve the low cost and mass production demands is to build the device on a single semiconductor substrate. Unfortunately, it is not an easy task to build a three axis vector magnetometer on a single chip as most of the common sensors only detect parallel or perpendicular field components. To solve this problem, two or more substrates are often aligned at right angles to each other, and then packaged together, but this unfortunately increases cost and size.
There are various techniques that have been disclosed to build two-axis sensors using all of the above mentioned sensing devices. Unfortunately, these techniques cannot measure the magnetic field component perpendicular to the plane of the substrate. Hall Effect devices have been built that provide all three axes through the use of permeable shields and sometimes through the use of a van der Pauw like technique for the in-plane components, but these devices are relatively high power and low sensitivity.
Another technique that has been proposed is to combine sensors that detect perpendicular components with those that detect parallel components, such as Hall Effect sensors with AMR, GMR, or MTJ sensors, but the difference in sensitivity, and possibly process incompatibility, makes this an unattractive solution.